This invention claimed hereafter is related to a process and apparatus developed for sewage treatment. More particularly, it is about a sewage treatment process and apparatus in which selenium (Se), specifically the 6-valent selenium, is effectively removed from sewage containing selenium.
In general, a wet-type flue gas desulfurization system is widely used for power-generating facilities, etc. In such flue gas desulfurization system smog is generally contacted with an absorbent slurry (e.g., limestone slurry). Sulfur oxides are then absorbed while the formed gypsum is separated from the slurry in the absorber as a by-product. However, sewage discharged from such flue gas desulfurization system can contain hazardous materials carried over from the smog and dissolved in the sewage. The hazardous materials must be removed from the sewage before it is discharged or recycled. Particularly, sewage from flue gas desulfurization system used for a coal boiler has a maximum selenium content of 10 milligrams per kg. coal, currently causing an environmental problem. Therefore, in order to provide harmless sewage discharges, it is desired that the sewage is treated to have its selenium content removed.
Note that selenium has a toxicity similar to arsenic compounds. Therefore, reports about disabilities cases have brought about imposition of emission regulations overseas. In Japan, since February 1994, selenium has become a newly regulated item, in conformity with the following standard: 0.01 milligram per liter under the environmental standard, 0.1 milligram per liter under the emission standard, and 0.3 milligram per liter under the standard for the amount precipitated into reclaimed land. In addition, selenium exists in the 4-valent state (primary form: selenious acid SeO.sub.3.sup.2-) and in the 6-valent state (primary form: selenic acid SeO.sub.4.sup.2-). It should be noted that the 6-valent selenium has a high solubility (95% at 20.degree. C.) and dissolves easily in water.
Actual flue gas desulirization systems are equipped with sewage treatment apparatus in which the sewage is cleaned. However, the conventional sewage treatment apparatus uses a process consisting of aggregation and precipitation, COD (Chemical Oxygen Demand) treatment, and denitrification treatment using microorganisms, respectively. Treatment for the reduction of selenium is not considered.
Conventional process for the removal of 4-valent selenium contained in water and the like is available. In this process the 4-valent selenium is separated (solvent extraction) into solid and liquid by means of treatment agents such as FeCl.sub.3, Fe.sub.2 (SO.sub.4).sub.3, chelate (e.g., trade name: Epolas MX-7, manufactured by Miyoshi Resin), or polymer heavy metal collector (e.g., trade name: Epoflock L-1, manufactured by Miyoshi Resin). However, an effective process for sewage treatment to remove the 6-valent selenium has not yet been found.
For this reason, when selenium (particularly the 6-valent selenium) contained in sewage, desulfurized sewage, etc., and is, for example, diluted with a large amount of water, an elaborate and costly back-end processing must be carried out to meet the above-mentioned standard for the dissolved amount.